Help with a mask & scroll animation

Hi all,
I’m looking to create an animation like the one linked below, and I’ve been down bad.

It’s going under the ‘Work We’ve Done’ section on my page. I’ve been trying to use a combination of containers as masks to hide content, and overlaying sticky containers to control opacity on certain containers.

One of the things I can’t figure out is how to scroll thru the content in the frame without scrolling the rest of the page behind it. Is there a way to stop a container from sticking at a certain point, instead of waiting for it to scroll out of view?

Anything helps! Thanks guys,

Here is my public share link: